Bimodal acrylate pressure-sensitive adhesive compounds

ABSTRACT

The invention relates to a polyacrylate which as a result of a polymerization process has a broad, bimodal molecular weight distribution. These acrylate pressure-sensitive adhesives are preferably processed from the melt. The low molecular weight fraction lowers the flow viscosity, while the high molecular weight fraction results in the achievement of a high shear strength, following appropriate crosslinking, for the acrylate pressure-sensitive adhesive.

This is a 371 of PCT/EP2003/013167 filed 24 Nov. 2003 (internationalfiling date).

The invention relates to a process for preparing a polyacrylate havingan at least bimodal molecular weight distribution, to a polyacrylateobtainable by this process, and to its use.

BACKGROUND OF THE INVENTION

Within the field of pressure-sensitive adhesives (PSAs), ongoingtechnological developments in the coating process mean that there is acontinual need for new developments. In the industry, hotmelt processeswith solvent-free coating technology are of increasing importance forthe preparation of PSAs, since the environmental regulations arebecoming ever greater and the prices of solvents continue to rise.Consequently, solvents are to be eliminated as far as possible from themanufacturing operation for PSA tapes. As a result of the associatedintroduction of the hotmelt technology, the requirements imposed on theadhesives are becoming every more stringent. Acrylate PSAs in particularare the subject of very intensive investigations aimed at improvements.For high-end industrial application preference is given topolyacrylates, on account of their transparency and weatheringstability. As well as these advantages, however, these acrylate PSAsmust also meet exacting requirements in respect of shear strength andbond strength. This profile of requirements is matched by polyacrylatesof high molecular weight and high polarity with subsequent efficientcrosslinking. The drawback with these high-shear-strength, polar PSAs,however, is that they are unsuited to the operation of hotmeltextrusion, since, as a result of the high flow viscosity, highapplication temperatures are necessary and, moreover, the molecularweight of the polymer is reduced by shearing within the extruder. Thisdamage significantly lowers the level of adhesive performance. The bondstrength and the tack are generally low, since the glass transitiontemperature is relatively high because of the polar fractions in theadhesives. The shear strengths in particular of hotmelt-coated acrylatePSAs drop significantly in comparison to the original solvent-coatedPSA. At the present time, therefore, various concepts are beinginvestigated with the aim of reducing flow viscosity and hence offacilitating the extrusion coating of these PSAs.

One very important concept is the targeted adjustment of the molecularweight distribution for the purpose of improved processing. Bimodalmolecular weight distributions assist easier processing, since lowmolecular weight fractions lower the flow viscosity while high molecularweight fractions raise the shear strength. Bimodal molecular weightdistributions are generally produced by means of targeted blending. InU.S. Pat. No. 5,548,014 polyolefin blends are prepared by this methodand processed in a hotmelt process. The general drawback of this methodis that in two process steps the polymers, first, must be prepared, withtheir different average molecular weight, and subsequently must bemixed. A process for such blending of polyolefins is described in U.S.Pat. Nos. 5,863,665 and 5,773,155. It is also necessary here tointroduce a relatively large amount of energy in order to mix thesystems with one another, since the miscibility of polymers with oneanother is relatively poor, because of their long polymer chains.

Furthermore, bimodal molecular weight distributions have been disclosedfor starch degradation in polyacrylate dispersions (U.S. Pat. No.6,084,018, U.S. Pat. No. 6,080,813, U.S. Pat. No. 5,705,563). There isno connection here, though, neither with the elastomer nor with anoperation of hotmelt extrusion.

U.S. Pat. Nos. 4,619,979 and 4,843,134 describe a preparation processfor the solvent-free polymerization of acrylates. Here, in a specificpolymerization reactor, highly branched polymers were prepared.Drawbacks are the high gel fraction formed during the polymerization,which although allowing a bimodal molecular weight distribution makes itimpossible to coat such a material, and the low conversion of thepolymerization, resulting in the need to remove acrylate monomers fromthe system again, an operation which is relatively costly andinconvenient. Since acrylate PSAs are generally composed of two or morecomonomers, and these comonomers possess different boiling temperaturesand vapor pressures, this is a very costly and inconvenient process.

It is an object of the invention to avoid the drawbacks which exist inthe prior art and to provide a broad, bimodally distributed polyacrylatehaving good hotmelt processing properties while retaining good adhesiveproperties. The polyacrylate ought preferably to have a low residualmonomer content.

This object is achieved by a polymerization process according to themain claim, and also by a polyacrylate and its use according to thecoindependent claims. Advantageous developments of the invention arecharacterized in the dependent claims.

SUMMARY OF THE INVENTION

The invention accordingly provides a process for preparing apolyacrylate having an at least bimodal molecular weight distributionand distinguished by the fact that a monomer mixture which comprises

-   a1) acrylic acid and/or acrylic esters of the formula    CH₂═C(R′)(COOR²), where R′=H or CH₃ and R² is a linear, branched or    cyclic alkyl chain having 1 to 20 carbon atoms,-   at 70%-100% by weight, based on the polymer,-   a2) olefinically unsaturated monomers containing functional groups,    at 0-30% by weight, based on the polymer,-   is polymerized in an at least two-phase free-radical polymerization    to give a polyacrylate having a broad, at least bimodal molecular    weight distribution, polymerization taking place in a first phase of    the at least two-phase polymerization, by means of a low initiator    concentration relative to the monomer, to give a first polymer    having a molecular weight which is high on average, and, before the    monomer mixture has been completely consumed by reaction, a next    phase of polymerization is started, by the addition at least once of    a regulator, and in this further phase or further phases a further    polymer is synthesized having a molecular weight which is relatively    low on averge.

DETAILED DESCRIPTION

By a broad, bimodally distributed polyacrylate is meant one in which apolymer or a molecular weight population (P₁ hereinafter) with arelatively low average molecular weight and a polymer or a molecularweight population with a relatively high average molecular weight (P₂hereinafter) are present alongside one another, so that the maxima ofthe two molecular weight distributions are preferably at least 50 000g/mol apart. In a preferred version the polymerization is configured intwo phases and the resulting polymer possesses two pronounced molecularweight peaks in the gel permeation chromatogram, i.e., two pronouncedmaxima in the molecular weight distribution that are least 50000 g/molapart.

In one preferred embodiment the polydispersity of the polymers isgreater than 6.

The at least two-phase free-radical polymerization is preferably takento a total conversion of all polymerization phases of greater than 97%,so that the residual monomer content becomes very low.

The composition of the corresponding monomers is chosen such that theresultant adhesives possess properties of pressure-sensitive adhesion inaccordance with D. Satas [Handbook of Pressure Sensitive AdhesiveTechnology, 1989, VAN NOSTRAND REINHOLD, New York].

The polymerization can be carried out in polymerization reactors whichin general are provided with a stirrer, two or more feed vessels, refluxcondenser, heating and cooling and are equipped for operation under N₂atmosphere and superatmospheric pressure.

The free-radical polymerization can be carried out in the presence of anorganic solvent or in mixtures of organic solvents. It is preferred touse as little solvent as possible. Depending on conversion andtemperature, the polymerization time amounts to between 6 and 48 h. Inthe case of solution polymerization, solvents used are preferably estersof saturated carboxylic acids (such as ethyl acetate), aliphatichydrocarbons (such as n-hexane or n-heptane), ketones (such as acetoneor methyl ethyl ketone), special-boiling-point spirit or mixtures ofthese solvents.

For the inventive process the compounds used as polymerizationinitiators are customary radical-forming compounds such as peroxides andazo compounds, for example. Initiator mixtures, too, can be used. Forone preferred version of the invention the molar ratio of initiator tomonomer in the first phase is less than 0.005, more preferably less than0.003. The addition of initiator in the first phase may take place inone step or in two or more steps. Initiators used with particularpreference are azoisobutyronitrile (AIBN) or Vazo 67™ (DuPont).

In the second phase, regulators are added to the polymerization to lowermolecular weight.

Examples of what are called polymerization regulators that can be addedinclude alcohols, ethers, dithioethers, dithiocarbonates,trithiocarbonates, nitroxides, alkyl bromides, thiols, TEMPO(2,2,6,6-tetramethylpiperidine N-oxy) and TEMPO derivatives. In oneparticularly preferred embodiment of the invention, isopropanol is usedas a regulator. The regulator is added no earlier than after one hour'spolymerization time but no later than 2 h before the end of reaction.The molecular weight distribution can be controlled with the point intime of the addition. The later the regulator is added, the lower thelow molecular weight fraction of the polyacrylate becomes. The amount ofregulator is guided by the efficiency, use being made of at least 0.01weight fractions based on the monomers. For the particularly preferredregulator isopropanol, use is made of between 3 and 30, more preferablybetween 5 and 25, weight fractions of isopropanol, based on themonomers.

Additionally it may be of advantage that for the purpose of increasingconversion an initiator is added that possesses a crosslinkingefficiency of more than 5. Examples of such initiators include Perkadox16™ from Akzo Nobel.

The object of the invention is further achieved by means of a newpolyacrylate having a broad, at least and preferably bimodal, molecularweight distribution, in which the molecular weight maxima in themolecular weight distributions of at least two polymers are preferablyat least 50000 g/mol apart. On the basis of its properties, thepolyacrylate of the invention is especially suitable for use in meltcoating processes and hence for the production of PSA tapes and PSAarticles. It exhibits a good shear strength and a flow viscosity atmoderate temperatures which is sufficient for hotmelt processing.

It may further be of advantage to blend the polymers of the inventionwith crosslinkers. Crosslinkers which can be used here are alldifunctional or polyfunctional compounds whose functional groups areable to enter into a linking reaction with the polyacrylates,particularly addition-polymerization, polycondensation or polyadditionreactions. These reactions will preferably engage at a carboxyl group.Suitable crosslinkers are, in particular, epoxides or isocyanates havingat least two functional groups, but also all other carboxyl-reactivecompounds. Metal chelate compounds can also be used for this purpose.

In an advantageous embodiment of the invention, therefore, thepolyacrylate contains olefinically unsaturated monomer units withfunctional groups in a fraction of 0-30% by weight, based on thepolymer. These groups are used to control the adhesive properties.

As vinyl compounds or olefinically unsaturated monomers containingfunctional groups it is possible in particular to use the following:vinyl esters, vinyl ethers, vinyl halides, vinylidene halides, vinylcompounds with aromatic rings and heterocycles in α position, examplesbeing vinyl acetate, vinylformamide, vinylpyridine, ethyl vinyl ether,vinyl chloride, vinylidene chloride, and acrylonitrile. It is preferred,moreover, to use monomers containing the following functional groups:hydroxyl, carboxyl, acid amide, isocyanato or amino groups.

As vinyl compounds containing functional groups it is also possible withgreat advantage to use compounds of the following structure:

where R₁═H or CH₃ and the radical —OR₂ represents or includes thefunctional group of the PSA and does not serve as a functional group forcrosslinking—with the base formed from b).

Particularly preferred examples of the vinyl compounds containingfunctional groups that are to be used for the purposes of the inventionare hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethylmethacrylate, hydroxypropyl methacrylate, allyl alcohol, maleicanhydride, itaconic anhydride, itaconic acid, acrylamide, benzylacrylate, benzyl methacrylate, phenyl acrylate, phenyl methacrylate,t-butylphenyl acrylate, t-butylphenyl methacrylate, phenoxyethylacrylate, phenoxyethyl methacrylate, 2-butoxyethyl methacrylate,2-butoxyethyl acrylate, dimethylaminoethyl methacrylate,dimethylaminoethyl acrylate, diethylaminoethyl methacrylate,diethylaminoethyl acrylate, cyanoethyl methacrylate, cyanoethylacrylate, 6-hydroxyhexyl methacrylate, N-tert-butylacrylamide,N-methylolmethacrylamide, N-(butoxymethyl)methacrylamide,N-methylolacrylamide , N-(ethoxymethyl)acrylamide,N-isopropylacrylamide, vinylacetic acid, tetrahydrofurfuryl acrylate,β-acryloyloxypropionic acid, trichioroacrylic acid, fumaric acid,crotonic acid, aconitic acid, and dimethylacrylic acid, this enumerationnot being exhaustive.

Further it is possible to use aromatic vinyl compounds, where preferablythe aromatic nuclei are C₄ to C₁₈ and may also include heteroatoms.Particularly preferred examples are styrene, 4-vinylpyridine,N-vinyl-phthalimide, methylstyrene, 3,4-dimethoxystyrene, and4-vinylbenzoic acid, this enumeration not being exhaustive.

For the preparation of PSAs the polymers are further blended,optionally, with resins. Resins which can be used are, for example,terpene resins, terpene-phenolic resins, C₅- and C₉-hydrocarbon resins,pinene resins, indene resins, and rosins, alone and also in combinationwith one another. In principle, however, it is possible to use all ofthe resins that are soluble in, the corresponding polyacrylate;reference may be made in particular to all aliphatic, aromatic, andalkylaromatic hydrocarbon resins, hydrocarbon resins based on singlemonomers, hydrogenated hydrocarbon resins, functional hydrocarbonresins, and natural resins.

In addition it is possible to add plasticizers, various fillers (forexample, carbon black, TiO₂, solid or hollow beads of glass or othermaterials, nucleators), expandants, compounding agents and/or aginginhibitors.

In one advantageous development, UV photoinitiators are added to thecopolymers. Useful photoinitiators are benzoin ethers, such as benzoinmethyl ether and benzoin isopropyl ether, substituted acetophenones,such as 2,2-diethoxyacetophenone (available as Irgacure 651 from CibaGeigy), 2,2-dimethoxy-2-phenyl-1-phenylethanone,dimethoxyhydroxyacetophenone, substituted alpha-ketols, such as2-methoxy-2-hydroxypropiophenone, aromatic sulfonyl chlorides, such as2-naphthylsulfonyl chloride, and photoactive oximes, such as1-phenyl-1,2-propanedione 2-(O-ethoxycarbonyl)oxime, for example.

The polyacrylates of the invention are coated directly from solutiononto a carrier material. In one particularly preferred embodiment thepolyacrylates are freed from the solvent and processed further from themelt. Concentration is carried out using with particular preference atwin-screw extruder, which is operated corotatingly orcounterrotatingly.

As carrier material, for adhesive tapes for example, it is possible hereto use the materials that are customary and familiar to the skilledworker, such as films (polyesters, PET, PE, PP, BOPP, PVC), nonwovens,foams, woven fabrics and woven films, and also release paper (glassine,HDPE, LDPE). This enumeration is not intended to be exhaustive.

The hotmelt PSAs of the invention are crosslinked by brief UVirradiation in the range of 200-400 nm using commercially customaryhigh-pressure or medium-pressure mercury lamps with an output of, forexample, 80 to 200 W/cm, or ionizing radiation, such as by electron-beamcuring, for example. For UV crosslinking it may be appropriate to adaptthe lamp output to the belt speed or, with the belt running slowly, toshade off part of the belt, in order to lower the thermal load thereon.The irradiation time is governed by the construction and output of therespective lamps.

For PSA coated from solution, the solvent is removed in a drying tunnelat elevated temperatures. The energy introduced can additionally be usedfor thermal crosslinking.

The invention further provides for the use of a pressure-sensitiveadhesive comprising the polyacrylate of the invention.

The invention additionally provides for the use of the resultantpressure-sensitive adhesive for an adhesive tape, the acrylatepressure-sensitive adhesive being present in the form of a single-sidedor double-sided film on a carrier.

EXAMPLES

The following exemplary experiments are intended to illustrate thecontent of the invention without wishing, through the choice of theexamples, to restrict the invention unnecessarily.

Test Methods

The polyacrylate compositions and their crosslinked products werecharacterized by the test methods set out below.

Shear Strength (Test A)

A strip of the adhesive tape, 13 mm wide, was applied to a smooth,cleaned steel surface. The application area measured 20 mm×13 mm(length×width). Subsequent procedure was as follows:

Test A: At room temperature a 1 kg weight was fixed to the adhesive tapeand the time taken for the weight to fall off was recorded.

Determination of the Gel Fraction (Test B)

The carefully dried, solvent-free adhesive samples are welded into apouch of polyethylene nonwoven (Tyvek web). The difference in the sampleweights before and after extraction with toluene gives the gel index, inother words the toluene-insoluble weight fraction of the polymer.

180° Bond Strength Test (Test C)

A strip 20 mm wide of an acrylate PSA coated onto a polyester wasapplied to steel plates. The PSA strip was pressed onto the substratetwice with a 2 kg weight. Immediately thereafter the adhesive tape waspeeled from the substrate at 300 mm/min and at an angle of 180°. Thesteel plates were washed twice with acetone and once with isopropanol.The results of the measurement are reported in N/cm and are averagedfrom three measurements. All measurements were carried out at roomtemperature under controlled-climate conditions.

Gel Permeation Chromatography (Test D)

The average molecular weight M and the polydispersity PD were determinedin the eluent THF used with 0.1% by volume trifluoroacetic acid.Measurement took place at 25° C. The precolumn used was PSS-SDV, 5 u,10³ A, ID 8.0 mm×50 mm. Separation was carried out using the columnsPSS-SDV, 5 u, 10³ and also 10⁵ and 10⁶ each with ID 8.0 mm×300 mm. Thesample concentration was 4 g/l, the flow rate 1.0 ml per minute.Measurement was made against PMMA standards.

Determination of the Conversion (Test E)

The conversion was determined by gas chromatography and is reported as apercentage relative to the amount by weight of the monomers used. Theresidual monomers were determined via GC, with a calibration curve beingset up for the individual monomers.

Determination of the Dynamic Flow Viscosity (Test F)

The measurements were carried out using the dynamic stress rheometerinstrument from rheometrics. For sample preparation the liquid polymerswere applied to a siliconized release paper carrier and dried in adrying oven at 120° C. for 10 minutes. The application rate was 100g/m². Strips were then cut and were laminated one on top another untilthe assembly has a thickness of about 1 mm. From these laminates,circular specimens with a diameter of 25 mm were cut out and therheological measurements were carried out using these laminates. At 130°C. the frequency was varied from 0.1 to 100 rad/s. For comparison, theflow viscosities measured in each case at 1 rad/s are reported.Measurement took place with a parallel plate arrangement.

Samples Investigated

The samples used for the experiments were prepared as follows.

Example 1

A 2 L glass reactor conventional for free-radical polymerizations wascharged with 20 g of acrylic acid, 380 g of 2-ethylhexyl acrylate, 133 gof special-boiling-point spirit 69/95 and 133 g of acetone. Afternitrogen gas had been passed through the reaction solution with stirringfor 45 minutes, the reactor was heated to 58° C. and 0.2 g of Vazo 67™(DuPont) was added. Subsequently the external heating bath was heated to75° C. and the reaction was carried out constantly at this externaltemperature. After a reaction of 2.5 h the batch was diluted with 100 gof acetone. After a reaction time of 4 h a further 0.2 g of Vazo 67™ wasadded. After a polymerization of 5 h the batch was diluted with 100 g ofacetone, and after 6 h with 100 g of special-boiling-point spirit 60/95.After a reaction of 24 h the polymerization was terminated and thereaction vessel was cooled to room temperature. The polymer was analyzedby test methods D and E. Thereafter the polymer was freed from thesolvent in a drying oven at 80° C. and then applied from the meltthrough a slot die at 50 g/m² to a Saran-primed PET film, cured withelectron beams, with an acceleration voltage of 230 kV, and thenanalyzed for adhesive performance using test methods A, B, and C.

Example 2

A 2 L glass reactor conventional for free-radical polymerizations wascharged with 20 g of acrylic acid, 380 g of 2-ethylhexyl acrylate, 133 gof special-boiling-point spirit 69/95 and 133 g of acetone. Afternitrogen gas had been passed through the reaction solution with stirringfor 45 minutes, the reactor was heated to 58° C. and 0.2 g of Vazo 67™(DuPont) was added. Subsequently the external heating bath was heated to75° C. and the reaction was carried out constantly at this externaltemperature. After a reaction time of 1 h, 20 g of isopropanol wereadded. After 2.5 h the batch was diluted with 100 g of acetone. After areaction time of 4 h a further 0.2 g of Vazo 67™ was added. After apolymerization of 7 h the batch was diluted with 100 g ofspecial-boiling-point spirit 60/95, and after 22 h with 100 g ofacetone. After a reaction of 24 h the polymerization was terminated andthe reaction vessel was cooled to room temperature. The polymer wasanalyzed by test methods D and E. Thereafter the polymer was freed fromthe solvent in a drying oven at 80° C. and then applied from the meltthrough a slot die at 50 g/m² to a Saran-primed PET film, cured withelectron beams, with an acceleration voltage of 230 kV, and thenanalyzed for adhesive performance using test methods A, B, and C.

Example 3

A 2 L glass reactor conventional for free-radical polymerizations wascharged with 60 g of acrylic acid, 380 g of 2-ethylhexyl acrylate, 133 gof special-boiling-point spirit 69/95 and 133 g of acetone. Afternitrogen gas had been passed through the reaction solution with stirringfor 45 minutes, the reactor was heated to 58° C. and 0.2 g of Vazo 67™(DuPont) was added. Subsequently the external heating bath was heated to75° C. and the reaction was carried out constantly at this externaltemperature. After a reaction time of 1 h, 60 g of isopropanol wereadded. After a reaction time of 4 h a further 0.2 g of Vazo 67™ wasadded. After a polymerization of 7 h and after 22 h, the batch wasdiluted with 100 g of acetone each time. After a reaction of 24 h thepolymerization was terminated and the reaction vessel was cooled to roomtemperature. The polymer was analyzed by test methods D and E.Thereafter the polymer was freed from the solvent in a drying oven at80° C. and then applied from the melt through a slot die at 50 g/m² to aSaran-primed PET film, cured with electron beams, with an accelerationvoltage of 230 kV, and then analyzed for adhesive performance using testmethods A, B, and C.

Example 4

A 2 L glass reactor conventional for free-radical polymerizations wascharged with 20 g of acrylic acid, 380 g of 2-ethylhexyl acrylate, 133 gof special-boiling-point spirit 69/95 and 133 g of acetone. Afternitrogen gas had been passed through the reaction solution with stirringfor 45 minutes, the reactor was heated to 58° C. and 0.2 g of Vazo 67™(DuPont) was added. Subsequently the external heating bath was heated to75° C. and the reaction was carried out constantly at this externaltemperature. After a reaction time of 1 h, 100 g of isopropanol wereadded. After a reaction time of 4 h a further 0.2 g of Vazo 67™ wasadded. After a polymerization of 22 h, the batch was diluted with 100 gof acetone. After a reaction of 24 h the polymerization was terminatedand the reaction vessel was cooled to room temperature. The polymer wasanalyzed by test methods D and E. Thereafter the polymer was freed fromthe solvent in a drying oven at 80° C. and then applied from the meltthrough a slot die at 50 g/m² to a Saran-primed PET film, cured withelectron beams, with an acceleration voltage of 230 kV, and thenanalyzed for adhesive performance using test methods A, B, and C.

Example 5

A 2 L glass reactor conventional for free-radical polymerizations wascharged with 20 g of acrylic acid, 380 g of 2-ethylhexyl acrylate, 133 gof special-boiling-point spirit 69/95 and 133 g of acetone. Afternitrogen gas had been passed through the reaction solution with stirringfor 45 minutes, the reactor was heated to 58° C. and 0.2 g of Vazo 67™(DuPont) was added. Subsequently the external heating bath was heated to75° C. and the reaction was carried out constantly at this externaltemperature. After a reaction of 2 h the batch was diluted with 100 g ofacetone, and after 3 h with 100 g of special-boiling-point spirit 60/95.After a reaction time of 4 h a further 0.2 g of Vazo 67™ was added, andafter a reaction time of 5 h 60 g of isopropanol were added. After apolymerization of 22 h the batch was diluted with 200 g of acetone.After a reaction of 24 h the polymerization was terminated and thereaction vessel was cooled to room temperature. The polymer was analyzedby test methods D and E. Thereafter the polymer was freed from thesolvent in a drying oven at 80° C. and then applied from the meltthrough a slot die at 50 g/m² to a Saran-primed PET film, cured withelectron beams, with an acceleration voltage of 230 kV, and thenanalyzed for adhesive performance using test methods A, B, and C.

Example 6

A 2 L glass reactor conventional for free-radical polymerizations wascharged with 20 g of acrylic acid, 380 g of 2-ethylhexyl acrylate, 18 gof isopropanol and 248 g of acetone. After nitrogen gas had been passedthrough the reaction solution with stirring for 45 minutes, the reactorwas heated to 58° C. and 0.2 g of Vazo 67™ (DuPont) was added.Subsequently the external heating bath was heated to 75° C. and thereaction was carried out constantly at this external temperature. Aftera reaction of 2.5 h the batch was diluted with 100 g ofacetone/isopropanol (93/7). After a reaction time of 4 h a further 0.2 gof Vazo 67™ was added. After a reaction time of 5 h and after 6 h thebatch was diluted with 100 g of acetone/isopropanol (93/7) each time.After a reaction of 24 h the polymerization was terminated and thereaction vessel was cooled to room temperature.

The polymer was analyzed by test methods D and E. Thereafter the polymerwas freed from the solvent in a drying oven at 80° C. and then appliedfrom the melt through a slot die at 50 g/m² to a Saran-primed PET film,cured with electron beams, with an acceleration voltage of 230 kV, andthen analyzed for adhesive performance using test methods A, B, and C.

The results are described below.

Table 1 shows, for all examples, the conversions and also the resultsfrom the gel permeation chromatography.

TABLE 1 Conversion M_(w) [%] [g/mol] M_(w)/M_(n) M_(P1)[g/mol]M_(P2)[g/mol] Example Test E Test D Test D Test D Test D 1 1.7 5985004.6 767000 — 2 2.0 575000 5.4 411000 690000 3 2.1 503000 11.7 356000802500 4 1.2 440500 14.8 388000 730500 5 2.0 615500 11.2 462000 852000 62.0 455000 4.9 624000 — M_(w) average molecular weight M_(w)/M_(n)polydispersity M_(P1) molecular weight peak in GPC for populations 1 and2 respectively

Example 1 is the reference and does not have a bimodal molecular weightdistribution. In contrast, Examples 2 to 5 were prepared in a secondphase with an additional regulator. The regulator added was isopropanol.In Examples 2 to 4 the point in time of addition was kept constant, atone hour after the start of reaction. In Example 5 the isopropanol wasadded after 4 hours from the beginning of reaction. Example 6 was runconventionally with a constant regulator fraction, and has a monomodalmolecular weight distribution. As a result of the increased regulatorfraction, however, the average molecular weight, at 455000 g/mol, isrelatively low here as well.

In comparison to the references, the polyacrylates prepared in twostages have a very broad bimodal molecular weight distribution. Thepolydispersity M_(w)/M_(n) is in all cases above that of the references(Examples 1+6). Moreover, as the isopropanol regulator added goes up,there is a fall in the average molecular weight. The exception isExample 5, since in this case, because of the later addition ofregulator, only the polydispersity rises. Examples 2-5 each exhibit twopeaks in the GPC spectrum, with the peaks being situated significantlyapart. Hence the criterion of a bimodal distribution is met.

The conversions in all examples are above 97%.

Table 2 below lists the adhesive properties of the individual examplesand compares them with one another.

TABLE 2 EB dose Gel index [%] HP 10 N RT [min] BSS [N/cm] Example [kGy](Test B) (Test A) (Test C) 1 40 40 1355 4.0 2 40 37 1280 4.6 3 70 381370 4.8 4 100 35 1195 5.0 5 50 42 1645 4.9 6 90 39 255 4.4 HP: holdingpower BSS. bond strength steel EB. electron beams

In order to attain optimum crosslinking, crosslinking was carried outwith different doses. The lower the average molecular weight of the PSA,the higher must be the applied EB dose in order to attain optimum andefficient crosslinking. In this way it is possible to generate PSAspossessing shear strength. Examples 1 to 5 in Table 2 demonstrate thatthe differences in terms of shear strength are very small if it isensured that the gel index is situated on a level. Conversely thedifferences in the bond strengths are significantly greater, since inthis case—probably as a result of the greater low molecular weightfraction—the adhesion to the substrate increases. Example 6 shows asignificantly lower shear strength, probably because the high molecularweight fraction is too low for a cohesive acrylate PSA.

For coating as a hotmelt the dynamic flow viscosity is critical. For allof the examples, therefore, for comparison, the flow viscosity wasmeasured using the rheometer. The results are set out in Table 3.

TABLE 3 η [Pa s] at 1 rad/s and 130° C. Example (Test F) 1 15525 2 102453  8475 4  6690 5 11425 6 12480 η: flow viscosity at 1 rad/s

As a result of the monomodal molecular weight distribution the flowviscosity of Example 1 is at a very high level. In comparison to this,the flow viscosity in Examples 2 to 5 falls off markedly as a result ofthe broad bimodal distribution. These adhesives can therefore beprocessed much more simply by a hotmelt operation, since thetemperatures that must be employed are lower and the polymers receiveless damage. Furthermore, with these pressure-sensitive adhesives, avirtually identical profile of adhesive properties is achieved, thebroader molecular weight distribution being compensated by irradiationwith a higher EB dose. Example 6 shows that, as a result of the loweraverage molecular weight, there is likewise a fall in the flowviscosity, but in terms of adhesive properties (particularly the shearstrength) no longer matches the broad bimodally distributed PSAs.

1. A process for preparing a polyacrylate having an at least bimodalmolecular weight distribution, which comprises polymerizing a monomermixture of a1) 70% to 100% by weight acrylic acid and/or acrylic estersof the formula CH₂═C(R′)(COOR²), where R′═H or CH₃ and R² is an alkylchain having 1 to 20 carbon atoms, and a2) 0 to 30% by weightolefinically unsaturated monomers containing functional groups, in an atleast two-phase free-radical solvent polymerization in the presence ofan organic solvent or in mixtures of organic solvents to give apolyacrylate having an at least bimodal molecular weight distribution,the polymerization being carried out in a first phase of the at leasttwo-phase polymerization, in the presence of a first initiatorconcentration, to give a first polymer having a first molecular weightand, before the monomer mixture has been completely consumed by thepolymerization, a further phase or phases of polymerization is or arestarted, by the addition at least once of a regulator, and in thisfurther phase or further phases a second polymer or polymers is or aresynthesized, said second polymer having a second molecular weight whichsecond molecular weight is lower than said first molecular weight. 2.The process of claim 1, wherein the at least two-phase free-radicalpolymerization is taken to a total conversion of all phases of greaterthan 97%.
 3. The process of claim 1 wherein the polymerization iscarried out in two phases and a bimodal molecular weight distribution isbuilt up, the molecular weight maxima in the molecular weightdistributions of the two polymers being at least 50000 g/mol apart. 4.The process of claim 1, wherein the polydispersity of the polymers isgreater than
 6. 5. The process of claim 1, wherein the molar ratio ofinitiator to monomer in the first phase is less than 0.005.
 6. Theprocess of claim 1, wherein the addition of initiator takes place in twoor more steps.
 7. The process of claim 1, wherein said at least oneregulator is selected from the group consisting of alcohols, ethers,dithioethers, dithiocarbonates, trithiocarbonates, nitroxides, alkylbromides, thiols, TEMPO and TEMPO derivatives.
 8. The process of claim1, wherein the regulator is added no earlier than after one hour'spolymerization time but no later than two hours before the end ofpolymerization.